In a typical transaction, a consumer may drive a vehicle up to a fuel dispenser in a fueling environment. The consumer arranges for payment, either by paying at the pump, paying the cashier with cash, using a credit card or debit card, or some combination of these methods. The nozzle is inserted into the fill neck of the vehicle and fuel is dispensed into the gas tank of the vehicle. Displays on the fuel dispenser track how much fuel has been dispensed as well as a dollar value associated with the fuel that has been dispensed. The customer relies on the fuel dispenser to measure the amount of fuel dispensed accurately and charge the customer accordingly. One method customers sometimes use to control costs is to pay for a preset amount of fuel based on a dollar or volume amount. Regulatory requirements, namely Weights & Measures, require that these customers receive all of the fuel for which they have paid to a highly accurate degree.
Operating behind the scenes of this process are valves that open and close the fuel flow path and a flow meter that measures the amount of fuel dispensed. The purpose of the flow meter is to measure accurately the amount of fuel that is being delivered to the customer so that the customer may be billed accordingly and inventory tracking may be undertaken. As noted, for preset dollar or volume transactions, the consumer relies on the flow meter to measure the fuel dispensed so as to know when to terminate the fuel flow. Some meters are inferential meters, meaning that the actual displacement of the fuel is not measured. Inferential meters have some advantages over positive displacement meters. Chief among these advantages is that inferential meters typically are smaller than positive displacement meters. One example of an inferential meter that may be used is described in U.S. Pat. No. 5,689,071, entitled “WIDE RANGE, HIGH ACCURACY FLOW METER.” The '071 patent describes a turbine flow meter that measures the flow rate of a fluid by determining the number of rotations of a turbine rotor located inside the flow path of the meter.
As fluid enters the inlet port of the turbine flow meter in the meter of the '071 patent, the fluid passes across two turbine rotors, which causes the turbine rotors to rotate. The rotational velocity of the turbine rotors is sensed by pick-off coils. The pick-off coils are excited by an a-c signal that produces a magnetic field. As the turbine rotor rotates, the vanes on the turbine rotors pass through the magnetic field generated by the pick-off coils, thereby superimposing a pulse on the carrier waveform of the pick-off coils. The superimposed pulses occur at a repetition rate (pulses per second) proportional to the rotors' velocity and hence proportional to the measured rate of flow.
A problem may occur when using a turbine flow meter. When fuel flows across the rotors, the rotors acquire some rotational momentum. When the fuel flow stops, the rotational momentum causes the turbine rotors to continue to rotate, despite the absence of fuel flow. This continued movement causes the turbine flow meter to continue generating measurement signals as if fuel were still flowing. The control system that receives the measurement signals from the pick-off coils of the turbine flow meter continues to register fuel flow falsely.
A solution to the aforementioned problem must be found to use a turbine flow meter as an accurate flow meter in a fuel dispenser. The present invention provides a solution to this problem.
The fact that not all valves that open and close the fuel flow path are well suited for preset cost or preset volume transactions is also of concern when designing fuel dispensers. Typically, to assist consumers in dispensing a fuel amount corresponding to the preset amount, some fuel dispensers are equipped with a two stage valve that allows high flow conditions throughout the majority of a fueling transaction and slow flow conditions at the terminating portion of the transaction. In slow flow conditions, the rate of fuel being dispensed slows dramatically to enable the dispenser to hit the predetermined volume or desired monetary amount. The slow flow portion of a preset transaction generates a consistent flow-rate so that the two stage valve may be de-energized at the proper time to achieve the desired termination point. In this manner, the consumer may stop squeezing the nozzle handle at the appropriate time when the desired amount of fuel is dispensed. To date, the two-stage valves that achieve the slow flow and high flow conditions work reasonably well, but may not be optimized to interact with inferential flow meters. Thus, any solution that improves the use of an inferential flow meter should also address this concern.